In an engine used for an automobile or the like, supercharging is performed by a supercharger to improve output of the engine. The supercharging of the air includes compressing the air by a compressor and then supplying the compressed air to the engine.
There are different types of superchargers, such as an exhaust gas turbine supercharger called as a turbocharger, and a mechanical supercharger which is mechanically powered and called as a mechanical supercharger. The turbocharger rotates the turbine using energy of the exhaust gas from the engine and compresses the intake air in a compressor directly coupled to the turbine to supply the compressed air to the engine.
Herein, the turbocharger uses the exhaust gas of the engine and thus may not be able to perform the supercharging sufficiently depending on a condition of the exhaust gas such as when the rotation speed of the engine is low. Therefore, as a measure against insufficient supercharging, an electric supercharger is known, in which an electric motor is provided so as to rotate the compressor by both the exhaust gas energy and the electric motor or solely by the electric motor.
To reduce the size of the electric supercharger, it is preferable to arrange the electric motor outside of a bearing for supporting the rotation shaft on the compressor side. The electric supercharger rotates the compressor using the electric motor. However, this causes the rotation shaft to rotate at a high speed and thus, it is necessary to provide a measure for reducing the shaft vibration.
FIG. 7 is a schematic view of a shaft supporting structure of a conventional electric supercharger.
In the shaft supporting structure of the conventional electric supercharger 101 shown in FIG. 7, a rotation shaft 102 extends outside a compressor wheel 104 attached to the rotation shaft 102. A compressor cover 106 covers components such as the compressor wheel 104.
On an extending part of the rotation shaft 102 extending outside the compressor wheel 104, a motor rotator 108 and a motor 110 are provided.
The motor rotator 108 is a rotator of the electric supercharger, which is fixed to the rotation shaft 102 to rotate with the rotation shaft 102. The motor 110 is a stator provided opposing the motor rotator in a direction perpendicular to the rotation shaft 102. Thus the motor 110 surrounds the motor rotator 108 and applies a rotation force to the motor rotator 108. The electric motor 107 is formed by the motor rotator 108 and the motor 110.
Bearings 112 are provided on both sides of the electric motor 107, i.e. on both sides of the motor rotator 108. By providing the bearings 112 in this manner, the extending part of the rotation shaft 102 has its center of gravity between the bearings 112. This structure allows the heavy motor rotator 108 to be supported at both ends thereof. Further, bearing housings 114 are provided for each covering the bearing 112. The bearings housing 114 is configured to supply oil to an interior of the bearing housing 114 and to drain the oil to an exterior of the bearing housing 114.
The shaft supporting structure for the electric supercharger shown in FIG. 7 supports the heavy motor rotator 108 at both ends thereof. This has a beneficial effect on reducing shaft vibration of the electric supercharger. However, this requires bearings 112 on both sides of the motor rotator 108, which leaves issues such as ease of assembling and a precision of centering the bearing housings.
In view of this, there is another shaft supporting structure which has an advantage as far as the ease of assembling and the precision of centering the bearing housings.
FIG. 8 is a schematic view of another shaft supporting structure of a conventional electric supercharger.
The same reference numerals are given in FIG. 8 without adding explanations for those configurations that are the same as those in FIG. 7.
In the shaft supporting structure for the electric supercharger shown in FIG. 8, the motor rotator 108 is supported by the bearing 112b on one end such that the motor rotator 108 overhangs from the bearing 112b. 
The shaft supporting structure for the electric supercharger shown in FIG. 8 has the motor rotator 108 overhanging from the bearing (the bearing 112b). This structure is advantageous in the ease of assembling and in the precision of centering the bearing housings. However, there is still an issue in the shaft vibration reduction. Especially in the attempt of increasing the output of the compressor (increasing the capacity of the supercharger), the weight of the motor rotator increases. Thus the measure for reducing the shaft vibration is desired.
Patent Literature 1 discloses to devise a shape of the bearing so as to reduce the shaft vibration in the shaft supporting structure for the electric supercharger whose motor rotator overhangs from the bearing.